I will definitely be giving it a try as soon as I can. Right now you can't easily go from React Native to web and you can't be 100% certain that mobile apps written in React Native will always work on Android and iOS because some RN APIs are only implemented for either platform. With ReactXP, they only expose the RN APIs that are common to both platforms, which gives a much higher confidence level for true cross platform reuse of app code.

Well if you aren't ignorant of documentation you can do perfect cross platform with RN. There is some APIs that are platform specific, because they actually are the thing user would expect in that platform(i.e. date pickers, switches, input fields and so on). Instead of providing user with 100% identical apps you provide platform specific which is the true meaning of "native feel".

I would use ReactXP for very small scale projects or prototypes. Why? Because imagine twitter sharing its app view layer onto a website, it wouldn't look nice. Small single page apps might work(Instagram maybe could be candidate for such experiment), big web apps tend to have different structures for web and mobile for a reason. Web user is different from mobile, therefore experience should be different. :)

It is nice touch with adding WinOS, you can do this with ReactNative as well, just need some manual(fairly easy) implementation. :)